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Chapter 64 · Conclusions – M. D. Ries ⁴⁰¹

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Summary

Total knee arthroplasty (TKA) is an effective treatment for severe arthrosis of the knee with predictable pain re- lief and longevity of the arthroplasty. However, late me- chanical failure may occur,necessitating revision surgery, and normal range of motion and kinematics are not reli- ably achieved. Current developments in surgical tech- nique, instrumentation, and implant design including computer-assisted navigation, minimally invasive surgery, use of more durable bearing surface materials, and kinematic implant designs, offer the potential to fur- ther improve the function and longevity of TKA.

Current Status of Total Knee Arthoplasty

Reliable improvement in pain and function can be ex- pected following TKA, and survivorship rates of 90%- 95% after 10 years have frequently been reported [1-5].

Early failures may result from infection, instability, malalignment, stiffness, reflex sympathetic dystrophy, and patellar problems. Many of these problems can be avoided by proper surgical technique, implant selection, appropriate postoperative pain management, and reha- bilitation. Late failures more typically occur from poly- ethylene wear, and less commonly from implant loosen- ing or periprosthetic fracture. However, elimination of gamma-irradiated in air UHMWPE has reduced the fre- quency of wear-related failures after TKA.

Although pain and function are dramatically im- proved after TKA, range of motion may still be slightly limited, causing impairment in activities which require greater knee flexion such as descending stairs or sitting in a low chair. Getting up from a seated position often re- quires use of the upper extremities to assist active knee extension.While these limitations may not be apparent to patients with relatively sedentary lifestyles, more active patients can experience some functional limitations after TKA.

New Bearing Surface Materials

Failures of fixed-bearing total knee tibial components have usually resulted from fatigue-wear mechanisms which were associated with gamma-irradiated in air UHMWPE. However, gamma-irradiation in air steriliza- tion has not been used by most manufacturers since the mid 1990s, and the typical failure mechanisms previous- ly observed do not appear to be continuing. Use of high- ly cross-linked polyethylene in total hip arthroplasty is as- sociated with a significant reduction in wear in simulator and early clinical studies [7-12]. However, cross-linking also reduces the mechanical properties of UHMWPE, which limits its utility in high contact stress applications such as fixed-bearing TKA [13, 14]. Impingement-related failures associated with a small contact area between the femoral neck and the acetabular rim have been reported in total hip arthroplasty [15, 16]. The bearing surface con- tact area in the knee is smaller than in the hip and con- tact stresses are an order of magnitude greater in the knee than in the hip [17]. Currently available highly cross- linked polyethylenes which have reduced mechanical properties compared with non-cross-linked polyethylene may therefore not be suitable for use as an alternative bearing surface in TKA.

During in vivo use, roughening of the cobalt-chrome counterface may also develop. In vitro wear studies of conventional as well as highly cross-linked polyethylene demonstrate an increase in wear when tested against a roughened counterface [18-20]. While wear may be re- duced by the use of highly cross-linked polyethylene, its mechanical properties are also reduced. A more appro- priate solution to minimize wear caused by in vivo coun- terface roughening without reducing UHMWPE me- chanical properties may be achieved with use of a scratch- resistant ceramic counterface rather than highly cross-linked polyethylene [21, 22].

Minimally Invasive Surgery

TKA performed through a conventional medial parap- atellar approach is associated with reliable pain relief,im-

64 Conclusions

M. D. Ries

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provement in function, and 90%-95% 10-year survivor- ship [1-5]. However, many patients experience significant pain and inflammation which typically occurs to some extent for 6 months after arthroplasty and may limit par- ticipation in rehabilitation exercises. Minimally invasive surgery permits TKA to be performed with less soft-tis- sue trauma.Early reports indicate that minimally invasive surgery is associated with reduced blood loss, less pain, and earlier return of quadriceps function and range of motion [23-25]. However, a minimally invasive approach may compromise surgical exposure and result in an in- crease in complications. Minimally invasive total hip arthroplasty has been reported to have a higher compli- cation rate than a standard approach [26]. By using small cutting blocks and avoiding dissection of the suprapatel- lar pouch, reliable results can be achieved with a compli- cation rate which is not greater than conventional TKA [23-25].Particularly when combined with a pre-operative patient education program and multimodal postopera- tive pain management, TKA performed through a mini- mally invasive approach appears to offer significant ad- vantages compared with conventional TKA. However, more muscular patients, those with prior surgery, stiff- ness, poor skin vascularity, or significant deformity re- quiring soft-tissue releases may not be appropriate can- didates for a minimally invasive approach.

Computer-assisted Surgery

Optimal axial (varus-valgus) alignment, component ro- tation, anteroposterior positioning, and implant sizing are important technical goals of TKA. Malalignment can compromise the function of the arthroplasty. Conven- tional intramedullary femoral and intra- or ex- tramedullary tibial instrumentation generally provides satisfactory axial alignment. Direct visualization of the posterior condylar line, epicondylar axis, and anteropos- terior axis of the distal femur permits proper rotational orientation of the femoral component.Intraoperative siz- ing of the distal femur and proximal tibia as well as flex- ion and extension gaps provides information for implant sizing and positioning.However,many series of TKA per- formed with conventional instrumentation include occa- sional cases of implant malalignment which may be suf- ficient to cause early failure of the arthroplasty or com- promise knee function.

Reliable improvement in implant alignment can be achieved with computer-assisted surgery (CAS) to orient the cutting blocks or combined with robotics to perform the bone cuts [27-30]. While most surgeons achieve sim- ilar implant alignment with conventional instrumenta- tion, CAS offers a benefit in reducing the number of out- liers with occasional malalignment. Although methods of CAS are evolving, currently available systems permit

navigation with either additional fluoroscopic or CT imaging, or imageless techniques [28-30]. CT provides accurate three-dimensional anatomical imaging of the knee, but this method requires preoperative CT scanning and templating of the CT-obtained images.Fluoroscopic- based navigation relies on intraoperative two-dimen- sional imaging and specific magnification calibration to maintain accuracy of the technique. With the imageless technique, anatomical data points are obtained intraop- eratively and a corresponding three-dimensional knee model is chosen by the computer, which matches the data points as closely as possible from a computer library.Each method requires placement of fixation pins in the tibia and femur, and bone and instrument registration with the CAS system.With additional fluoroscopic or CT imag- ing, further improvement in accuracy may be achieved compared with imageless techniques, although the rela- tive accuracy of these three methods has not been well de- fined.With any CAS system,additional surgical time is re- quired, as are more nursing and surgeon training and greater costs to implement the system.Availability of CAS in TKA is limited by these issues at many institutions.

However, the potential benefits of improvement in im- plant alignment may outweigh the additional surgical time and cost required for CAS,particularly as more user- friendly and less expensive systems are developed.

New Implant Designs

Incomplete restoration of high-demand functions fol- lowing TKA is not surprising, since there are many anatomical differences between the normal and the re- placed knee. One or both cruciate ligaments and both menisci are removed, the joint-line position is altered, and the articular geometry of the bearing surfaces is changed.Fluoroscopic kinematic studies of TKA patients consistently demonstrate “paradoxical” motion patterns in which the femur is positioned posteriorly on the tibia during knee extension and moves anteriorly during knee flexion. This pattern of motion is the reverse of the nor- mal knee kinematic pattern in which the femur moves posteriorly during flexion. Abnormal kinematics after TKA may limit flexion and decrease quadriceps efficien- cy. However, kinematics after unicompartmental knee arthroplasty, in which most of the normal anatomical constraints (anterior cruciate ligament, contralateral tibiofemoral compartment, and patellofemoral joint) are preserved, are much closer to that of the normal knee.

Knee motion and function after unicompartmental knee arthoplasty also compare favorably to TKA. However, ef- forts in the past to retain both cruciate ligaments during TKA have resulted in high failure rates due to stiffness and implant loosening.Alternatively, guided-motion implant designs are intended to provide more normal kinematics

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without retention of the anterior cruciate ligament. The posterior cruciate-substituting cam-and-post mecha- nism is an example of guided motion, since the engage- ment of the cam and post during flexion limits anterior translation of the femur and results in a less abnormal knee kinematic pattern than posterior cruciate-retaining TKA [31].Variation in the tibial bearing surface geometry can also affect tibiofemoral kinematics [32]. Therefore, further improvements in implant design may permit more normal knee kinematics and better knee function, particularly for higher demand activities.

The Future of Total Knee Arthroplasty

TKA is an effective, well-established treatment for severe arthrosis of the knee. However, further improvement in the longevity of the arthroplasty can be achieved with more durable bearing surface materials. Due to the non- conforming shape of the bearing surface, hard-on-hard bearings are unlikely to have a role in TKA and UHMW- PE will remain an important bearing surface material.

However, improvements in manufacturing and elimina- tion of gamma-irradiation in air sterilization have already resulted in fewer wear-related problems. Although cur- rently available highly cross-linked polyethylenes de- crease the mechanical properties of UHMWPE, which limits their use in TKA, further modifications which im- prove polymer wear but retain mechanical properties may have an important role in the future.

The introduction of minimally invasive surgery has been well accepted by the general population of arthritic patients, despite the fact that the risks and benefits of the procedure have not been well defined. However, the ben- efits of earlier return of motion and better pain control may result in improvement in long-term function after TKA. Continued refinement of instrumentation, surgical technique, and indications should result in more pre- dictable outcomes, so it is likely that minimally or less in- vasive surgery will become a standard technique with well-defined indications and that “conventional’ surgical approaches will also have specific indications.

Although computer-assisted surgery offers improve- ment in implant alignment, the current barriers of in- creased surgical time, cost, and training limit its applica- tion. However, further developments should overcome these barriers to make the techniques of computer navi- gation more readily available. The extent of implementa- tion of navigation in routine surgical practice is likely to depend on the success of developing user-friendly sys- tems such as non-optical tracking systems which provide reproducible alignment.

Biological treatments to restore arthritic joint sur- faces including autologous cartilage transplantation, use of stem cells, and synthetic cartilage may eventually be-

come viable alternatives to TKA. However, continued re- search will be required for some time before the efficacy of these treatments is defined.

TKA is one of the most cost-effective treatments in medicine, and the number of joint replacement proce- dures performed in the United States and Europe has been growing steadily in recent years. With the baby- boom population now reaching an age at which arthritic symptoms typically develop,it appears likely that consid- erably more patients will benefit from TKA in the future.

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